The Neutrino: A Guide To The Invisible Particle That Has Astronomers So Excited

This month astrophysicists found new evidence of dark matter —
the so far invisible substance that scientists believe makes up
80% of our known universe.

The astrophysicists detected strange X-ray emissions from
neighboring galaxies. They think the X-rays are from a decaying
neutrino — but an entirely new kind of neutrino that only exists
in theory.

Not only could they be the direct evidence of dark matter
scientists have hunted for 80 years, neutrinos are getting a lot
of attention in other fields. They could become the building
block for some incredible new technology.

Neutrinos are fundamental particles of matter that are created by
a specific type of radioactive decay, called beta decay, and from
nuclear reactions like the ones that take place in the sun and
nuclear reactors. Neutrinos were also produced during the Big
Bang.

Scientists know there are three different types of neutrinos —
the different kinds exist because the particles spin in different
patterns.

It seems like we know a lot about these particles, but really we
don't, considering we're basically swimming in them.

Neutrinos are hard to pin down because they have almost no mass
and interact weakly with other particles. It takes a giant
chamber, like the one pictured above, full of extremely hot
liquid hydrogen for scientists to be able to detect the presence
of these particles.

The discovery of neutrinos.

Though it has been theorized since 1930, we didn't discover the
neutrino (in physical reality) until 1953. But its discovery was
fundamentally important — it saved one of the most basic laws of
science: The law of conservation of energy.

During beta decay, a neutron turns into a proton and spits out an
electron. When scientists first measured what happens during beta
decay they ran into a problem: The electrons were emitted with
less energy than the reaction started with. It looked like energy
was being lost somewhere in the beta decay process.

According to the law of conservation of energy, any reaction has
to produce as much energy as it starts with. The scientists could
not figure out an explanation for it, and some even considered
giving up on the law.

But in 1930 the physicist Wolfgang Pauli came up solution, one
that was considered insane for the time. He theorized that there
must be another particle being emitted, it was just a particle
that no one could see or detect.

According to Jayawardhana, Pauli had a great sense of humor about
his crazy theory. He actually bet a case of champagne against
himself and the ability of anyone to be able to detect the
particle ever.

Finally, in the 1950s scientists confirmed that neutrinos exist.

Generally, neutrinos pass right through the ground. But a small
fraction of neutrinos will actually hit something — sending out
decay products that we can measure. There are several different
types of neutrino detectors, but the basic idea is to catch a
large number of neutrinos and hope that some of these will crash
into atoms.

The image on the right shows the first recorded observation of a
neutrino. The photograph was taken inside the chamber of a
neutrino detector in the Argonne National Laboratory outside
Chicago.

The spot where the invisible neutrino collides with a proton is
circled in red.

Types of neutrinos.

Scientists know there at least three different kinds of
neutrinos: electron, muon, and tau. The different types are
referred to as "flavors" and they correspond with the way the
particle is oscillating. Jayawardhana said the particles change
flavors all the time.

"It's as if chocolate flavored ice cream changed to vanilla, then
to strawberry, then back to chocolate — it really is that
strange," Jayawardhana said.

The scientists detected the particle by mixing cadmium chloride
in a tank of water. Cadmium absorbs neutrons really well. When
the neutrinos collide with protons they produce neutrons and
positrons. The positrons then collide with electrons and give off
gamma rays.

Now that scientists have built more sophisticated neutrino
detectors that can reliably detect all three flavors of these
particles and study them, Jayawardhana says neutrinos have tons
of potential.